This invention relates to reducing the 4,2'-disubstituted diphenyl carbonate content of isomeric mixtures containing 4,4'-disubstituted diphenyl carbonate and, optionally, decarboxylating the product to the corresponding ether and, more particularly, to a process for decreasing the 4,2'-disubstituted diphenyl carbonate content of certain isomeric mixtures of the carbonate, wherein the phenyl groups are mono o-, m- and p-substituted, by treating the mixture with the corresponding m-or p-substituted phenol and, optionally, a decarboxylating agent, and removing the o-substituted phenol reaction product produced to form product carbonate, or optionally, ether. In particular, if a substituted diphenyl carbonate in which each phenyl ring is mono nitro-substituted is made by nitrating diphenyl carbonate, the product consists of a mixture of dinitrodiphenyl carbonate isomers in which each ring is mono nitro-substituted essentially in the ortho and para position. In such mixtures the 4,2'-dinitrodiphenyl carbonate content can be decreased to give an essentially pure, 4,4'-dinitrodiphenyl carbonate, or optionally ether, by treating the mixture with p-nitrophenol, and optionally, a decarboxylating agent, and removing the more volatile and more soluble o-nitrophenol reaction product by volatilization or recrystallization from a solvent.
The commercial preparation of substituted diphenyl compounds such as the diphenyl ethers in which each ring is monosubstituted has become important because of the use of such materials, in particular, the diamino derivatives, in polyamides, polyimides, and polyamide-imides. Because of the multiplicity of regioisomers formable in the manufacture of such compounds and the difference in properties of materials made from the different isomers, it is of commercial importance to be able to cheaply produce regiopure compounds. In particular, it is important to be able to make regiopure 4,4'-compounds such as 4,4'-dinitrodiphenyl ether which is easily reduced to the 4,4'-diaminodiphenyl ether for use in the polymer industry to manufacture polyamides, etc.
A number of methods have been reported to make 4,4'-diaminodiphenyl ether, none of which is entirely satisfactory. One procedure starts with the nitration of chlorobenzene to chloronitrobenzene which can be purified to give the para isomer essentially regiopure. That material is then reacted with regiopure p-nitrophenol to form regiopure 4,4'-dinitrodiphenyl ether which is then reduced to the diamine. This method is taught by J. A. Oscar in U.S. Pat. No. 3,387,041 issued to Dupont. Alternatively, diphenyl ether may be nitrated and the other isomers separated from the 4,4'-isomer in what is a costly and somewhat ineffective procedure; a method which is taught in French Patent No. 2,129,235 issued to Rhone Poulanc.
Now it has been found that by employing the lability of the diphenyl carbonate systems regiopure 4,4'-dinitrodiphenyl carbonate can be made by starting with a regioimpure mixture of nitrated diphenyl carbonates by treating the apparently mobile carbonate system in the melt or solution with 4-nitrophenol and removing the 2-nitrophenol produced by displacement either by volatilization or recrystallization. Inclusion of a decarboxylating agent in the process gives the 4,4'-disubstituted diphenyl ether instead of the carbonate.